add the pure bash core: core, str, naming and tags

Ported from make/utils.mk, make/def.mk, make/apps/def.docker.mk and
make/apps/def.mk, with the examples that sat as dead comments in
make/apps/def.mk turned into actual assertions.

Two deviations from the make macros are pinned and documented in DELTAS.md;
both only show up on code paths nothing uses.

spec/unit/naming_resu_helper.sh

@@ -0,0 +1,5 @@
+#shellcheck shell=sh
+# Helper: myos_resu sets MYOS_RESU_NIAMOD/MYOS_RESU_PATH as a side effect;
+# a subshell would lose them, so the assertion runs them here.
+myos_resu "aya@github.com" >/dev/null
+printf '%s %s\n' "$MYOS_RESU_NIAMOD" "$MYOS_RESU_PATH"